This invention is concerned with a novel process for removing homogeneous catalyst group VIII metals from chemical process streams which comprises treating the process streams with organic sulfur compounds containing a carbon-sulfur double bond.
Homogeneous catalysts containing group VIII metals, such as Wilkinson's catalyst, chloro tris triphenyl-phosphine rhodium, RhCl(P.0..sub.3).sub.3, are commercially important in the chemical industry. Because of their inherent cost, it is economically important that as much of the metal as possible be easily recoverable without loss of product yield. It is also important, particularly in the pharmaceutical industry, that the products not be contaminated with the metal.
Heretofore, recovery of metals from homogeneous catalysts has involved ionic species or conversion to ionic species by oxidation such as with nitric acid and hydrogen peroxide. Also, polymeric sulfur containing resins have been used but these suffer from the disadvantage of being expensive and the need to prepare the resins as well as the need to recover the metal from the resin.
Other methods of recovery such as adsorption on activated carbon, silica gel or alumina present difficult, expensive recovery problems in that the metal is present in very low concentrations, usually measured in parts per million, and that the adsorbent is difficult to get rid of.
Furthermore, most of the known methods for recovery of homogeneous catalyst metals deal with recovery after the desired product has been removed by some prior isolation steps such as distillation, filtration and the like at which point the metal may be distributed among various filter cakes, filtrates and the like depending on the isolation procedures. None of the known methods is suitable for treating process streams still containing the desired product.
Now, with the present invention there is provided a novel process for removing homogeneous catalyst group VIII metals from chemical process streams which comprises treating the process streams with an organic sulfur compound containing a carbon-sulfur double bond, preferably, thiourea.
It is an advantage of the novel process that recovery of the metal can occur before the desired product is isolated and before the metal is dispersed among various fractions of the original process stream.
It is a further advantage of the novel process that the metal is recovered in high concentration. The organic sulfur compound/metal complex often contains the metal in higher concentration that did the original homogeneous catalyst.